1. Field of the Invention
This invention relates to a signal channel extending between an electronic device on a motherboard and an electronic device on a daughter card through a card edge connection, and, more particularly to a signal channel configuration having increased capacitance to electrical ground at the card edge connection to compensate for inductance introduced within a contact terminal forming part of the signal channel.
2. Description of the Related Art
Electrical connections are typically made between a motherboard and a daughter card through a card edge connection, with a number of contact pads extending in one or more rows along the edge of the daughter card being electrically connected to a similar number of contact terminals within a connector attached to the motherboard. It is often difficult to achieve a proper impedance for the a high-speed signal channel extending through such a connection because of the local inductance contributed by the contact terminal within the connector and because of the additional impedance associated with circuit traces adjacent the connector. When effective compensation is not applied for these sources of inductance, discontinuities in the line impedance of the channel cause reflections that degrade signal quality.
A conventional method for tuning the impedance of a signal channel is through the addition of a discrete component, such as a capacitor connected between the signal channel and electrical ground. What is needed is a method eliminating the additional cost and physical space on a circuit card required to provide for an additional discrete component.
U.S. Pat. No. 6,425,766 describes a connector system including a circuit card and method of manufacturing the circuit card. By way of example, the system includes a connector having an elongated housing with an elongated cardreceiving slot. A plurality of signal terminals and ground terminals are mounted on the housing along the slot and have contact sections extending into the slot. A circuit card has an edge insertable into the slot, a ground plane facing a surface of the card at least near the edge and a plurality of signal contact pads and a plurality of ground contact pads along the edge for engaging the contact sections of the signal terminals and the ground terminals, respectively. The size or area of the signal contact pads is varied relative to the ground contact pads to vary the capacitance in the area of the terminaltocircuit card interface and, thereby, vary the impedance of the system. What is needed is a system in which the impedance of the signal channels can be tuned without requiring that individual ground contact pads must be placed next to the individual signal pads for which the impedance is being tuned.
Methods for compensating for sources of inductance at an edge card connection include modifying the connector to be attached to the motherboard, as described, for example in U.S. Pat. No. 5,259,768, which describes a connector including body portions having mechanically nonfunctional sections of a given area, which effect a given capacitance. The mechanically nonfunctional sections are selectively trimmable to selectively vary the area thereof and thereby vary the capacitance of the terminals and, therefore, the impedance of the connector to match the given impedance of the electrical circuit. The connector includes a plurality of signal terminals and a plurality of ground terminals. The ground terminals have at least two points of contact for engaging a common ground circuit on the printed circuit board for reducing the inductance between a particular ground terminal and its respective circuit trace. What is needed is a method for modifying the circuit traces of the daughter card to eliminate the costs associated with making such specific modifications to the connector.
Other examples of card edge connections including double rows of contact pads are found in U.S. Pat. Nos. 5,061,190 and 6,234,807. What is needed is a way to provide such a card edge connection with simple and effective tuning of signal channel impedance.